1. Field of the Invention
The present invention relates to a projection apparatus for projecting an image of an original such as a microfilm, document, book or the like on a projection plane in a reader or copying machine.
2. Description of the Prior Art
In microfilm readers or reader-printers, an image recorded on a microfilm is projected on a screen through a projection lens in an increased magnification, this projected image being read as it is or further projected on a photosensitive member through the reader printer to obtain the copy thereof.
Since different rates of reduction are generally used when images are recorded on microfilms, the magnification of projection must be adjusted depending on the microfilms used.
The prior art variable magnification projection apparatus has such a disadvantage that when the magnification is changed to vary the rate of an optical path length across a lens, the distribution of light (illuminance) on the projected surface of a screen or photosensitive member will vary so that fluctuations are brought about with respect to the quality of image and the characteristics of the photosensitive member. In order to overcome this problem, a slit-exposure type copying machine has been proposed in which one of different slit plates is adapted to be inserted across the optical path on each change of magnification. Such a copying machines has, however, various disadvantages in that a mechanical structure, for example, the one for detecting the change of magnification, becomes more complicated and is increased in size, and in that the copying machine as a whole becomes more complicated since the shape of each slit plate must delicately be varied, and that the copying machine becomes expensive.
It has been also proposed that the distribution of light emission in an original illuminating lamp is changed depending on the change of magnification. However, this proposal does not provide the sufficient correction.
It has been also proposed that the distribution of illuminance on the projected surface be corrected by moving a condenser lens or lamp in connection with the change of magnification. Such a proposal also includes a disadvantage in that various mechanisms for detecting the changed magnification and for moving the lens or lamp in connection with the change of magnification will increasingly be complicated and increased in size as the number of selectable magnifications increases.
It has been further proposed that the magnification can be changed by selecting one of different lenses. In this case, it is required to utilize a so-called Kohler illumination for imaging the filament of an illumination lamp on the film-side pupil of a projection lens to effectively illuminate a group of exchangeable lenses. If the illumination is not proper, the light illuminating the projection lens may extremely be reduced in amount or unevenness may be brought about in the amount of light.
For such a reason, if the difference between one magnification and another magnification is smaller on projection, the distance of the film-side pupil from the surface of a film can be designed to be invariable for various different lenses to be used. If the difference in magnification is larger on projection, however, the pupil of the projection lens cannot be designed in the same manner. This also results in an increase in cost. A method is normally carried out in which the image of the filament in the illumination lamp is formed at the position of the film-side pupil in the projection lens by moving the condenser lens depending on the properties of the projection lens.
Such a method has, however, disadvantages in that a proper matching is hard to obtain since the position of the condenser lens is determined by eye-measurement, that an unskilled person may fail to match the position of the condenser lens to the property of the projection lens so that an obscure image will be observed with an effort, and that an unevenness may be produced in the density of copied images due to the unevenness of illumination to take wasteful copies in a printer.